CMU professor studies galaxy collision

When you think about the Milky Way colliding with another galaxy, the first thing that probably comes to mind is its predicted collision with our neighbor galaxy Andromeda. Nicknamed Milkomeda or Milkdromeda, the collision is not expected to occur for another four billion years, possibly long after terrestrial life can be supported on Earth and hopefully long after someone can think of a better name for it.

However, it’s thought that the Milky Way has a long history of colliding with dwarf galaxies, reshaping its structure each time. For example, the Milky Way’s collision with the nearby Sagittarius galaxy may be to thank for our galaxy’s signature spiral arms.

A team of researchers, including Carnegie Mellon University Physics professor Sergey Koposov, have discovered that approximately eight to ten billion years ago, the Milky Way experienced a head on collision with one galaxy in particular, possibly its largest collision known to date.

This dwarf galaxy, nicknamed the “Sausage” galaxy, did not survive the impact. New papers in the Monthly Notices of the Royal Astronomical Society explain that the team of researchers used data from the European Space Agency’s Gaia satellite, which maps the stellar content of our galaxy. This data showed that the collision left that dwarf galaxy’s stars moving in radial orbits with a general oval shape which led to the galaxy’s nickname.

“We plotted the velocities of the stars, and the sausage shape just jumped out at us,” said Wyn Evans, a researcher in Cambridge. “As the smaller galaxy broke up, its stars were thrown onto very radial orbits. These Sausage stars are what’s left of the last major merger of the Milky Way.”

After the collision between the two galaxies, the researchers believe that the path of the stars from the mutilated Sausage galaxy took them to the center of the Milky Way, creating the “bulge” of stars at our galaxy’s center and the surrounding stellar halo. “This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit,” said Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in a statement for the Simons Foundation. The researchers also believe that the Sausage galaxy is responsible for multiple globular clusters within the Milky Way.

The researchers plan to continue their study of the effects of this collision and the chemistry of the stars that came from the Sausage galaxy.